Metabolism — Protein & Starvation

Protein Metabolism

Amino acids

• Proteins are polypeptides created from amino acids linked together by peptide linkages
• 20 amino acids present in the body in significant quantities
• Of the 20, 10 are essenstial amino acids, which cannot be synthesized within the body in sufficient quantities and must be obtained, pre-formed, from food.
• Amino acids absorbed from diet
  • Absorbed by cells, especially hepatocytes
• Amino acids re-absorbed from filtrate in PCT

Protein anabolism

• Peptide linkages → covalent bonding of amino acid amine group and carboxyl group by condensation
• Proteins created in ribosomes in rough endoplasmic reticulum, according to translation of mRNA, which is itself transcribed from cell DNA
• Further post-translational processing of proteins occur in the smooth endoplasmic reticulum (e.g. glycosylation)

Protein storage

• After entering a cell, amino acids almost immediately converted into proteins, therefore intracellular amino acid concentration remains low
• Amino acids “stored” as proteins
• The liver stores large amounts of exchangeable proteins as rapidly available amino acid source

Protein Catabolism

• Proteins destined for degradation are “tagged” by another regulatory protein, ubiquitin, and targeted for degradation into it amino acid components by the proteosome in the cytosol
• Amino acids are recycled for formation of other proteins
• Use of proteins for energy
  Deamination → Release of NH₃ + NADH + H⁺
  • Ammonia released from deamination is removed from the blood by conversion to urea in the liver
    • 2 x NH₃ + CO₂ → Urea + H₂O
  • Deaminated amino acids can be used in the citric acid cycle
    • via Acetyl-CoA, ketogenesis or conversion to fatty acids
  • Deaminated amino acids can be used for gluconeogenesis

Regulation of Protein Metabolism

• Insulin and growth hormone promote storage of amino acids as proteins
• Glucocorticoids promote release of amino acids from protein depots
• Testosterone promotes storage of proteins in cells (esp. muscle)
• Thyroxine → increases metabolism
  • In presence of excess fat and carbohydrate → protein synthesis and storage
  • In fat and carbohydrate defecit → increased protein break-down

Starvation

• Daily obligatory protein loss
  • ~30g
• Except for obligatory protein loss, body utilizes carbohydrates and fats almost exclusively
• Once body carbohydrate and fat stores are depleted, amino acids are oxidized and used for energy
  • ~125g protein / day

Sakurai 2016

Answer by Sakurai